1. Field of the Invention
The present invention relates to interleaf paper for photosensitive printing plate material, and more particularly to interleaf paper for protecting a water-soluble oxidation preventing layer formed on the photosensitive printing plate.
2. Description of Related Art
In a conventional method of manufacturing a photosensitive printing plate, a metal support such as an aluminum plate is coated with a photopolymerizing mixture, which is composed of a chemical compound including an ethylene type double bond which can addition-polymerize, a photopolymerization starting agent, an organic high molecular compound, and a thermal-polymerization prohibiting agent. The photosensitive printing plate is covered with a film of a desired original image and is exposed, and thereby, the photopolymerizing mixture of exposed areas polymerizes and hardens. Then, the photopolymerizing mixture of unexposed areas is dissolved and removed. Thus, a hardened relief image is formed on the printing plate.
Japanese Patent Provisional Publication No. 55-118041, Japanese Patent Publication No. 61-19025 and Japanese Patent Provisional Publication No. 57-99647 disclose interleaf paper which protects the printing plate material. The interleaf paper aims to improve the cutting, the adhesion and the releasability.
Research on the high photosensitivity printing plate, which is made of photopolymerizing sensitive material, has further developed, and the printing plate has been is applied to a variety of applied fields. In particular, a laser direct plate making system conformable to a laser oscillation wavelength such as 488 nm of the argon ion laser and 532 nm of FD-YAG laser has already been put to practical use. To use the photopolymerization type as the printing plate material, a water-soluble polymer layer as an oxidation preventing layer is ordinarily formed on the printing plate material.
Recently, interleaf paper covering the above-mentioned printing plate has been material is automatically released. To use the printing plate material of the photopolymerization type for the laser direct plate making system, it is necessary to provide the interleaf paper with a performance conformable to the automatic interleaf paper releasing apparatus Examples of the automatic interleaf paper releasing apparatus are a machine which releases the interleaf paper by shifting it with a rubber roller, etc., a machine which releases the interleaf paper by sucking the interleaf paper with a sucking disc, etc. and a machine which blows the interleaf paper off by air pressure.
U.S. Pat. No. 4,336,093 relating to Japanese Patent Publication No. 57-23259 discloses an interleaf paper for covering the photosensitive printing plate material coated with the water-soluble oxidation preventing layer (water-soluble barrier layer to prevent oxygen), and the interleaf paper is coated with plastic so that the interleaf paper can be released by the automatic interleaf paper releasing apparatus by being sucked with the sucking disc, etc. of the apparatus.
When the printing plate material is covered with an interleaf paper coated with plastic, a high voltage is applied to the interleaf paper to charge it electrically and the interleaf paper electrostatically adheres to the printing plate material. Then, the interleaf paper is released when the printing plate material is used.
If the plastic coating layer of the interleaf paper adheres to the water-soluble oxidation preventing layer formed on the photosensitive printing plate, there is a problem in that it takes a long time until the sensitivity of the photosensitive printing plate is stabilized, since air hardly permeates the interleaf paper, that is, the air permeability of the interleaf paper is too low. The air permeability (ISO 3687-1976) represents a property of the paper against permeation of air, and it is expressed as the time which is required for 100 cm.sup.3 of air to pass through 645 mm.sup.2 of the paper.
On the other hand, if the air permeability of the interleaf paper is too high, the sucking disc, etc. of the automatic interleaf paper releasing apparatus cannot release the interleaf paper, since the photosensitive printing plate material as well as the interleaf paper is sucked. Hence, the interleaf paper which is coated with plastic is not conformable to a variety of automatic interleaf paper releasing apparatuses.